1. Field of the Invention
This invention relates generally to solar collectors and is concerned more particularly with a solar collector having more efficient means for conducting heat inwardly and transferring it to a fluid medium.
2. Discussion of the Prior Art
A solar collector of the tubular type generally includes a coaxial pair of inner and outer tubes having radially spaced wall portions. The outer tube usually is made of a material, such as glass, for example, which is transparent to solar energy. Generally, the inner tube has a blackened outer surface for absorbing impinging solar energy and converting it into thermal energy. Thus, a fluid, such as water, for example, may flow along the inner surface of the inner tube to extract energy therefrom and carry it to a thermal load.
The inner and outer tubes of the solar collector generally have respective opposing end portions sealed to one another; and the outer tube generally is provided with an exhaust tubulation for evacuating the annular space between the radially spaced wall portions of the tubes. The resulting vacuum space between the outer and inner tubes permits the passage of radiant solar energy, and minimizes heat losses due to convection and conduction from the outer surface of the inner tube. Consequently, the inner tube generally is made of glass also to provide ease in sealing opposing end portions thereof to respective opposing end portions of the outer tube, and to provide the inner an outer tubes with similar coefficients of thermal expansion.
The inner tube generally is designed to support the entire length of the outer tube, which may be considerable for a solar collector, and provide the rigidity required for maintaining the radial spaced relationship as well as protecting the integrity of the sealed end portions. Consequently, the inner tube generally has a thick wall to provide it with the necessary structural strength. However, the relatively thick wall of the inner tube reduces efficiency in conducting the heat energy from the coated outer surface to the inner surface.
Furthermore, a uniform flow of fluid through the inner tube generally is preferred in prior art solar collectors. However, the resulting laminar flow of fluid through the inner tube does not bring large portions of the fluid into contact with the inner surface of the inner tube for more efficiently transferring heat energy from the inner tube to the fluid.